Nowadays several medical image acquisition techniques and systems exist that render a digital signal representation of a medical image, e.g. a radiographic image.
One example of such a system is a computed radiography system wherein a radiation image is recorded on a temporary storage medium, more particularly a photostimulable phosphor screen. In such a system a digital signal representation is obtained by scanning the screen with radiation of (a) wavelength(s) within the stimulating wavelength range of the phosphor and by detecting the light emitted by the phosphor upon stimulation.
Other examples of computed radiography systems are direct radiography systems, for example systems wherein a radiographic image is recorded in a solid state sensor comprising a radiation sensitive layer and a layer of electronic read out circuitry.
Still another example of a computed radiography system is a system wherein a radiographic image is recorded on a conventional x-ray film and wherein that film is developed and subsequently subjected to image scanning.
Still other systems such as a tomography system may be envisaged.
The digital image representation of the medical image acquired by one of the above systems can then be used for generating a visible image on which the diagnosis can be performed. For this purpose the digital signal representation is applied to a hard copy recorder or to a display device.
Commonly the digital signal representation of the image is subjected to image processing prior to hard copy recording or display.
In order to convert the digital image information optimally into a visible image on a medium on which the diagnosis is performed, a multi-resolution image processing method (also called multi scale image processing method) has been developed by means of which the contrast of an image is enhanced.
According to this multi-resolution image processing method an image represented by an array of pixel values is processed by applying the following steps. First the original image is decomposed into a sequence of detail images at multiple scales and a residual image at a scale lower than the minimum of said multiple scales. Next, the pixel values of the detail images are modified by applying to these pixel values at least one non-linear monotonically increasing odd conversion function with a slope that gradually decreases with increasing argument values. Finally, a processed image is computed by applying a reconstruction algorithm to the residual image and the modified detail images, the reconstruction algorithm being such that if it were applied to the residual image and the detail images without modification, then the original image or a close approximation thereof would be obtained.
The above image processing technique has been described extensively in European patent EP 527 525, the processing being referred to as MUSICA image processing (MUSICA is a registered trade name of Agfa-Gevaert N.V.)
The described method is advantageous over conventional image processing techniques such as unsharp masking etc. because it increases the visibility of subtle details in the image and because it increases the faithfulness of the image reproduction without introducing artefacts.
Prior to being applied to a hard copy recorder or to a display device the image signal representing the radiation image (whether or not having been subjected to the above-described contrast enhancement processing) is subjected to a signal-to-density conversion according to a conversion curve, referred to as gradation (processing) curve.
The conversion of pixels to output values suitable for reproduction or display comprises the selection of a relevant sub-range and the conversion of data in this sub-range according to a specific gradation processing curve.
Preferably the relevant sub-range and the curve to be applied are adapted to the object and to the examination type so that optimal and constant image quality can be guaranteed.
Commonly one of a set of gradation curves, created and stored in advance, is selected upon entry of data concerning the type of image to be processed, e.g. upon entry of an indication on the examination type.
Due to the large number of degrees of freedom and the large number of parameters that can be set, many gradation curves suitable for different types of applications can be defined. However, storage of a large number of gradation curves demands for a lot of storage capacity.
In European patent specification EP 0 110 185 this problem has been dealt with. A method has been disclosed for obtaining a number of different gradation curves from one of about ten basic gradation curves by rotating and parallel shifting one of the basic gradation curves. The manipulation of the basic gradation curve depends on object parameters, radiation source parameters, examination type etc.
European patent 549 009 discloses a method wherein a gradation curve is defined by means of a set of image specific parameters. The gradation curve is deduced from a canonical form taking into account several parameters such as minimum and maximum signal level, maximum density etc. The canonical form as well as these parameters depend on the examination type of the read out image. The disclosed method thus requires input on the examination type of the read out image.
The state of the art workflow was commonly as follows.
A digital representation of a radiation image acquired by an image acquisition system as described higher is applied to an image-processing module together with a code indicative of the examination type. In the image processing module parameter settings for different examination types and examination sub-types, identifiable by means of a code, are stored in advance for example in the form of a look up table. Upon entry of a code, the parameters corresponding with the examination type identified by the code are retrieved from the look up table and the image processing including the gradation processing is controlled by the retrieved parameter settings.
Determining the optimal parameter values for each examination type is highly time-consuming because before a set of processing parameters can be accepted for a certain examination type, this set of parameters has to be evaluated on a large number of test images of that specific examination type.
The number of individual examination types is rather large since for each anatomic region (e.g. thorax) a number of examination subtypes exist (bed, paediatric, etc.).
Hence the total number of parameters to be determined is large.
In most large hospitals a centralised, radiology specific information system (RIS) is available so that the information on the examination type can be sent via this RIS.
If such a system is not available, the information on the examination type must be entered manually into an identification system. This kind of identification is more error prone than the RIS system.
In the former years the use of computed radiography systems was to a large extent limited to large radiography centres in the western market. Nowadays more smaller hospitals and private practices started to use digital systems. In addition also in non-industrialised regions computed radiography is gaining importance.
The adjustment and control of the image processing parameters demands for extensive knowledge of medical applications, which knowledge is often not available in or too expensive for these smaller and/or less industrialised centres.
The fact that in prior art methods the processing parameters had to be determined in advance and had to be fed to the processing apparatus thus constitutes a drawback of prior art methods.
Another drawback of the prior art methods is related to the detection of collimation borders.
In the prior art the selection of the relevant sub-range for conversion according to a gradation curve was performed on the basis of the image histogram. Pixels belonging to the collimation borders were not considered. Therefore it was required to detect the collimation borders in advance.
Because of the wide diversity of images and the complexity of this problem, a certain percentage of failures is inevitable (around 1%). As a consequence the correct operation of the detection of the relevant signal range is affected.
U.S. Pat. No. 5,164,993 discloses a method and apparatus for automatically and adaptively generating optimum tonescale transformations for digital radiographic images based on an analysis of the image histogram. An improvement of the method additionally uses information about the examination type being processed to produce a tonescale that is better customized for a given input.
This method still relies on a calculation of the collimated area, more specifically the border between the true direct x-ray background and the anatomy is determined.
This method results images with varying contrast as a function of the range of a region of interest. The obtained density and contrast in an image depends on the examination type.
U.S. Pat. No. 4,276,473 issued Jun. 30, 1981 also relates to the conversion of digital signal values representing a radiological image into corresponding density values. The level of the electric signal is converted in such a way that the maximum value of the electric signal corresponding to the maximum density of the radiation image is converted into a level resulting in a given optical density value of the reproduced image on the recording material. The minimum level corresponding to the minimum density is converted to a level resulting in the fog density of the recording material. Hence a signal range which is entirely dependent on the read out image is mapped onto a fixed density range. Consequently, when the latitude of the read out signal range varies, for example because the fysionomy of the exposed patients varies, the average contrast in the reproductions will also vary. There is no control of the variation of the contrast obtained in the reproductions of images with different information content.
U.S. Pat. No. 5,617,313 describes a method of gradation processing for displaying images of partial areas of radiation images. The described method does not provide fixed contrast.
U.S. Pat. No. 5,151,947 describes an image processing method wherein gradation processing conditions of a limited area are determined and wherein the gradation of the image is processed based upon the determined conditions. The described method results in a floating contrast and is not independent of the examination type.